LED illuminator apparatus, using multiple luminescent materials dispensed onto an array of LEDs, for improved color rendering, color mixing, and color temperature control

ABSTRACT

An LED array includes three or more strings of bare LEDs mounted in close proximity to each other on a substrate. The strings of LEDs emit light of one or more wavelengths of blue, indigo and/or violet light, with peak wavelengths that are less than 490 nm. Luminescent materials deposited on each of the LED chips in the array emit light of different wavelength ranges that are of longer wavelengths than and in response to light emissions from the LED chips. A control circuit applies currents to the strings of LEDs, causing the LEDs in the strings to emit light, which causes the luminescent materials to emit light. A user interface enables users to control the currents applied by the control circuit to the strings of LEDs to achieve a Correlated Color Temperature (CCT) value and hue that are desired by users, with CIE chromaticity coordinates that lie on, or near to the black body radiation curve. Preferably a transparent material is dispensed on the substrate between the LED semiconductor chips to substantially surround the LED semiconductor chips. Thereafter at least one layer containing luminescent materials is applied on the LED semiconductor chips and the transparent material.

BACKGROUND

The present invention relates generally to electrical lamp fixtures usedfor general-purpose lighting, and specifically to an improved lightemitting diode (LED) illumination apparatus, incorporating an LED arrayin which multiple phosphors have been dispensed onto the LEDs, in orderto improve the color rendering, color mixing, and color temperaturecontrol of the apparatus.

In order for LED illuminators and light engines to act as a satisfactoryreplacement for traditional general-purpose lighting, it is desirableand even necessary to produce white light with characteristics that aresimilar to the light produced from an incandescent bulb, or in somecase, to accurately replicate the light provided by natural sunlight.This is especially important for lighting applications that demand highquality light with well-controlled parameters, such as lighting forprofessional photography, videography, and the motion picture industry.In a general sense, this means that the LED illuminator or light engineshould have a spectral response or characteristic that mimics thespectral response of an incandescent bulb, and/or natural sunlight, atspecific color temperatures. The spectral characteristics of both theLED illuminator and the target light sources can be expressed in theform of a spectral plot of light emission as a function of wavelength,and also in terms of related measures including Correlated ColorTemperature (CCT), hue (which can be quantified using CIE chromaticitydiagram coordinates), and Color Rendering Index (CRI).

Briefly, the Correlated Color Temperature (CCT) of an illuminator orlamp is the color temperature of a black body radiator which to humancolor perception most closely matches the light from the lamp, and istypically expressed in degrees Kelvin (K). In practice it is primarilyapplicable to white light sources. A typical incandescent bulb will havea CCT in the range of 2500-3000K, typically referred to as “warm white”.Illuminators with higher CCT values may be described as “cool white”.Light from the sun may have CCT values in the 5000-6500K range,depending on time of day, the height of the sun above the horizon, andalso the degree of overcast. It is a highly desirable attribute for anLED illuminator to have a well-defined and controlled color temperature,with CCT values ranging from approximately 2500K to 6500K or evenhigher, depending on the application. LED illuminators may also providevariable color temperature, either through a finite number of CCTsettings, or via continuously varying control.

Color Rendering Index (CRI) provides a quantitative measure of a lightsource's ability to faithfully reproduce the colors of illuminatedobjects, in comparison to an ideal or natural light source. For thecomparison to be valid, the test light source and the reference sourcemust be of the same color temperature. For light sources above 5000K,daylight is used as the reference source. For light sources under 5000K,an ideal black body radiator of the same color temperature is used. Afull description of the measurement of CRI is beyond the scope of thisdocument. However, the basic measurement process consists of measuringthe light reflected from a series of test color samples, whenilluminated by the test light source and the reference light source. Inpractice, software packages that are provided with commerciallyavailable visible light spectrometers are able to compute the ColorRendering Index of light sources. In principle, natural sunlight willhave a CRI of 100, and the light emitted by an ideal black body radiatorwill also have a CRI of 100.

It is also possible to specify or quantify the hue or color of lightusing CIE chromaticity diagram coordinates. The color coordinates of anideal black body radiator, taken at different CCT values, arerepresented on a CIE chromaticity diagram as a curved line segment.However, it is important to note that the color coordinates of a lightsource do not provide any indication of the CRI of the light source.

LED illuminators that are intended to produce white light for generalillumination purposes, face two significant challenges. They shouldprovide light of the intended color temperature, generally in the rangeof 2500K to 6500K, depending on the desired appearance and application,with CIE chromaticity diagram color coordinates that lie on, or veryclose to, the black body radiation curve. What is meant by “very closeto” will be explained below. They should also achieve a high CRI, asclose to 100 as possible. This is especially important for demandingapplications such as in the fields of professional photography,videography, and motion picture filming. By using a mix of red, green,and blue LEDs, it is easy to provide any desired color temperature.However, the color rendering of such an RGB LED illuminator will be verypoor, with CRI values in the 70's, or even lower. This is due to thefact that the RGB LEDs have narrow bandwidth emission, with FWHM (fullwidth at half maximum) bandwidths of only 25-30 nm, for each of thethree LED colors/types. For example, test objects that reflectsignificant amounts of yellow wavelengths will not render accurately,due to an RGB LED illuminator's lack of emitted light in the yellowregion. Even if amber LED chips are added, there is a “dead zone” thatis roughly in the range of 550 to 590 nm in which LEDs have very lowemission efficiency, making it extremely difficult to obtain CRI valuesabove 92%, even when using a large number of LED wavelengths.

The most common method for obtaining good color rendering from an LEDilluminator is to coat blue LEDs with phosphors that absorb light energyfrom the blue LEDs, and convert a portion of this energy intobroad-spectrum emission at higher wavelengths, typically with a spectralpeak in the yellow region of the visible light spectrum. In thisdocument, higher wavelengths and longer wavelengths have identicalmeaning and are used interchangeably. Typical phosphors have FWHMbandwidths of approximately 50 to 120 nm, and therefore provide greaterspectral fill than individual LEDs. This approach can provide reasonablygood color rendering, with a fixed color temperature. However, it can bedifficult to accurately control the color temperature that results, andit may also be difficult to achieve lower color temperatures, such as“warm white”. For this reason, some prior art embodiments add red LEDs,as a means of “warming” the light output, and also potentially offeringthe ability to vary the color temperature of the illuminator. While theaddition of red LED chips provides advantages in terms of colortemperature control, the narrow spectral bandwidth of the added red LEDshas limited benefit in terms of color rendering, and in fact mayactually reduce the CRI of the light output as the output of the redLEDs is increased. The CRI of such an illuminator is determinedprimarily by the spectral characteristics of the phosphor that is usedto coat the blue LEDs.

Due to the limitations described above, there exists a need for an LEDilluminator that provides the combined light output from a cluster orarray of multiple LED chips, for applications that demand high-qualitylighting. In addition to providing the usual advantages of LED lighting,in terms of energy efficiency, long life, and reliability, it desirablyprovides a well-controlled Correlated Color Temperature (CCT),preferably with the ability to vary the CCT over a wide range via someform of user control. It preferably also provides extremely good colorrendering, with CRI values that exceed 95, and ideally achieve CRIvalues of 98 and above, throughout the illuminator's full range of CCTsettings. Finally, the light from the LED illuminator is preferablyhighly uniform, in terms of color and hue, over its field of view.

SUMMARY OF THE INVENTION

According to one embodiment, an illumination apparatus comprises an LEDarray, where the array includes three or more strings of LEDs. Thestrings of LEDs emit light of one or more wavelengths of blue, indigoand/or violet light, with peak wavelengths that are less than 490 nm.The LED array comprises bare LED chips that are mounted in closeproximity to each other on a substrate. Luminescent materials aredeposited on each of the LED chips in the array. The materials emitlight of different wavelength ranges that are of longer wavelengths thanlight emissions from the LED chips, in response to light emissions fromthe LED chips; A control circuit applies currents to the strings ofLEDs, causing the LEDs in the strings to emit light, which causes theluminescent materials to emit light. A user interface enables users tocontrol the currents applied by the control circuit to the strings ofLEDs to achieve a Correlated Color Temperature (CCT) value and hue thatare desired by users, with CIE chromaticity coordinates that lie on, ornear to the black body radiation curve.

According to one embodiment, a method is described for making anillumination apparatus. An array of unpackaged LED chips mounted on asubstrate is provided, the array including two or more strings of LEDchips, wherein said strings of LEDs emit light of different wavelengthranges. A transparent material is dispensed on the substrate between theLED semiconductor chips to substantially surround the LED semiconductorchips. Thereafter at least one layer containing luminescent materials isapplied on the LED semiconductor chips and the transparent material.

All patents, patent applications, articles, books, specifications, otherpublications, documents and things referenced herein are herebyincorporated herein by this reference in their entirety for allpurposes. To the extent of any inconsistency or conflict in thedefinition or use of a term between any of the incorporatedpublications, documents or things and the text of the present document,the definition or use of the term in the present document shall prevail.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the spectral profiles of ideal black body radiators, atseveral Correlated Color Temperature (CCT) values.

FIG. 2 is a representation of the prior art, showing the spectralprofile of a typical RGB LED illuminator.

FIG. 3 shows the corresponding CIE chromaticity diagram for the RGB LEDilluminator of FIG. 2.

FIG. 4 is a representation of the prior art, showing the spectralprofile of a typical LED illuminator using blue LEDs that are coatedwith a phosphor to produce white light.

FIG. 5 is a representation of the prior art, showing the spectralprofile of an LED illuminator that uses blue LEDs that are coated with aphosphor, with the addition of red LED chips to provide either a warmerwhite light, or potentially the ability to adjust the color temperatureof the light.

FIGS. 6A and 6B show two views of one embodiment of an LED array of thepresent invention, showing the use of multiple LED chips with multiplephosphor types, configured as multiple channels or strings of LEDs.

FIGS. 7A and 7B are a representation of one embodiment of the presentinvention using an array of LEDs with applied phosphors, showing anexploded view of the illuminator apparatus, without outer housing.

FIG. 8 is a representation of one embodiment of the present invention,showing additional details of the illuminator apparatus.

FIG. 9A is a representation of the spectral profile of one embodiment ofthe present invention, illustrating a spectral profile with very highCRI. FIG. 9B shows the corresponding CIE chromaticity diagram for thisilluminator.

FIGS. 10A and 10B are representations of one embodiment of an LED arrayof the present invention, showing a geometric configuration of twowavelengths of LED chips and three phosphor types, respectively, thatprovides for good color mixing and uniformity of the beam.

FIG. 11 shows a cross-section view of one embodiment of the LED array ofthe present invention, showing the LED chips in cross-section, prior tothe application of a transparent silicone gel and phosphors, depictingthe light emission characteristics of the LEDs.

FIG. 12 shows a cross-section view of one embodiment of the LED array ofthe present invention, showing the LED chips in cross-section, after theapplication of transparent silicone gel and phosphors, depicting theimproved light emission characteristics of the LEDs.

FIGS. 13A and 13B show schematic block diagrams of the electroniccircuit of two embodiments of the present invention, showing the abilityto separately control multiple strings of LEDs, thereby allowing controlof both the intensity and CCT of the illuminator's light output.

DETAILED DESCRIPTIONS OF THE PREFERRED EMBODIMENTS

The Color Rendering Index (CRI) of a light source provides aquantitative measure of the quality of the light provided by the source,in terms of its ability to accurately render the colors and appearanceof illuminated objects. High CRI values, approaching the ideal of 100,are important for light sources used in such demanding applications asprofessional photography, videography, and in the motion pictureindustry. Additionally, the Correlated Color Temperature (CCT) and hueof the light source are tightly controlled, preferably with the abilityto adjust or vary the CCT of the light source.

FIG. 1 shows spectral plots of an ideal black body radiator over theapproximate range of visible light, with a wavelength range of 400 nm to700 nm, at a variety of color temperatures. As can be seen from theplots, the spectral response of an ideal black body radiator is afunction of its CCT. Without getting into the mathematics of CRIcalculation, a light source of a given CCT will achieve very high CRI ifits spectral response closely matches that of an ideal black bodyradiator at that same CCT. Therefore, the plots shown in FIG. 1 can beviewed as objective or target spectral response curves for high qualitylight sources, at a variety of CCT values.

By blending or mixing the light output of red, green, and blue LEDs, itis possible to create light of almost any visible color or hue,including white light at a wide variety of CCT values, thereby mimickingthe appearance of black body radiators at the same CCT value. FIG. 2shows a representative spectral plot of a prior art RGB LED light sourcewith CCT of 4500K, showing the three narrow-band spectral peaks providedby the blue (201), green (202), and red (203) LEDs, respectively.

The CIE chromaticity diagram shown in FIG. 3 shows the color coordinatesof this RGB LED light source, showing that they fall on the idealblackbody radiator curve. The triangle within the diagram represents thefull color or hue gamut that is possible with the RGB LED light source.The curved line that runs in the middle of the chart represents theideal blackbody radiator curve, and the dot represents the CIEcoordinates of an ideal blackbody radiator with CCT of 4500K. Byappropriately proportioning the output of the RGB LEDs, the light outputcan be placed anywhere within the triangle represented by the threeindependent red, green, and blue light sources, and also at any CCTvalue along the black body curve. As such, the perceived hue or color ofthe light, when viewed directly, will also match the hue or color of anideal black body radiator of the same CCT value.

However, this RGB LED light source will provide very poor colorrendering. Qualitatively, in comparing the spectral plot of FIG. 2 withthe plot of an ideal black body radiator of the same 4500K CCT (204), itcan be seen that the spectral profile of the RGB LED illuminator hasmany deep gaps or voids in its spectrum, as well as spectral peaks. Thisresults in a low CRI value. Conversely, high quality light with high CRIcan be achieved by smoothing out the spectral profile, including fillingin the gaps in the spectrum, as well as attenuating any sharp peaks inoutput.

A common method for providing an LED light source with moderately goodCRI (above 80, for example) is to use blue LED chips that have beencoated with phosphors. Excitation of the phosphor material by the blueLEDs induces Stokes shift in the emission of light from the phosphor,resulting in an emission wavelength range that is at higher wavelengthsthan the excitation wavelength. The Stokes shift can range from a fewtens of nanometers, to as much as 200 nm or more. In LED illuminators,the excitation wavelength is typically in the range of 410 nm to 490 nm(with perceived colors ranging from violet or indigo to blue). Dependingon the phosphor material used, the major emission wavelength range maybe in the green, yellow, or red portions of the visible light spectrum.Said phosphor formulations are commercially available, and are designedto absorb energy at particular lower or shorter wavelengths (such as areemitted by blue LEDs), and to emit light at higher or longerwavelengths. The emissions from said phosphors have a much widerspectral bandwidth, typically with FWHM values of 50-120 nm, versus only25-30 nm for typical LEDs. Replacing green and red LEDs, each withnarrow emission bandwidth, with green, yellow, and/or red phosphors withmuch wider emission bandwidth, creates an overall emission spectrum thatprovides much higher CRI. Commercially available phosphors are typicallyavailable in the form of small particles or powders, with typicalmaterials being silicates, aluminates, garnets, and nitrides, withvarious attributes of emission wavelength range, efficiency, lifetime,etc. The phosphor material(s) are typically mixed into some form ofsilicone gel or epoxy, and then applied to the top surface of the LEDchip(s).

FIG. 4 shows a plot of a prior art LED illuminator, showing the directspectral output from blue LEDs (401), as well as the broader spectraloutput from the applied phosphors, primarily in the yellow region of thespectrum (402). With appropriately chosen phosphor materials and properapplication of the phosphor materials to the LEDs, this prior art lightsource can be designed to have the same 4500K CCT value as the prior artRGB LED light source of FIG. 2. Therefore, its CIE chromaticity diagramwill be essentially the same as the one shown in FIG. 3. However, itsspectral plot comes closer to matching the plot of an ideal black bodyradiator at the same 4500K CCT (403), with fewer deep gaps or voids inthe spectral plot, and therefore its CRI is substantially higher. Priorart LED illuminators using phosphors can achieve CRI values in the 80's,and sometimes in the low 90's.

Although the CCT (and hue) of an LED illuminator using blue LEDs andphosphors can be controlled by appropriately engineering the type ofphosphor material used, and the thickness and density of its applicationon top of the blue LEDs, it is difficult to achieve precise control. Itis also difficult to design such a light source with adjustable orvariable CCT. One prior art method for providing both tighter control ofCCT, as well as adjustability or variable CCT, is to add in a quantityof red LED chips, possibly with separate control of their drive current.By varying the proportioning of the light output from the blue LEDs withphosphors, and the red LEDs, it is possible to adjust the CCT of thecombined light output. FIG. 5 shows the spectral plot of such aconfiguration, showing the blue LED peak (501), the broader spectralprofile of the phosphor emission (502), and the red LED peak (503), alsocompared to the spectral plot of an ideal black body radiator of thesame 4500K CCT (504). The CIE chromaticity diagram for this prior artlight source will also be essentially the same as the one shown in FIG.3, with coordinates that lie on the ideal black body radiator curve. Byvarying the output of the red LEDs, the coordinates will move to otherCCT values along the curve. Although the addition of some red LEDs mayslightly improve the CRT value of the illuminator, it may also lead toexcessive peaks in the red portion of the spectrum, resulting in worseCRI values at lower CCTs (which require increased red LED output). Inaddition, the spatial positioning of the red LED chips within the lightsource can lead to “hot spots” of red light within the field of view,contributing to poor color/hue uniformity. It is also true that precisecontrol of the phosphor application is still required, in order toachieve CIE chromaticity diagram color coordinates that along, or closeto, the black body radiation curve.

Each hue or color of light has a coordinated point (x, y) in the CIEchromaticity diagram, as shown in FIG. 3. The black body radiation curveis a well defined curve in the diagram. While it is desirable for hue ofthe illumination light to be on the black body radiation curve in thediagram, for some lighting applications, it is adequate for the hue ofthe illumination light to be close to or near the black body radiationcurve, such as where the “near” or “close to” hue is defined as pointswhose x and y color coordinates are both within +\−0.006 of the x and ycoordinates of a point lying on the black body radiation curve, with thedesired or intended CCT value. Such and other variations are within thescope of the invention.

The present invention seeks to achieve very high CRI values, while alsoproviding well-controlled and adjustable/variable CCT and hue, withexcellent color mixing and uniformity. This is achieved by using one ormultiple wavelengths of blue (and/or violet or indigo) LEDs. In otherwords, where LEDs emitting light of multiple different wavelength rangesare used, the wavelength ranges may be in one or more of the blue,violet and indigo portion of the spectrum. All of the LEDs are coatedwith multiple types of phosphors having differing spectral profiles fortheir emissions. The peak wavelengths of the LEDs that serve as theexcitation source for the phosphor emissions, are at 490 nm and below.The phosphor emissions will typically be at wavelengths that are 20-250nm higher or longer than the excitation wavelength. Further, by havingindependent control of the drive current for different subsets of theLED chips and their respective phosphor types, the CCT of the lightsource can be easily adjusted or varied.

In addition to using traditional phosphor materials, it is also possibleto use other types of luminescent material, including quantum dots. Thekey attribute of quantum dots is that their emission spectra are afunction not only of the material from which they are made, but also ofthe size of the quantum dot particles. With appropriate selection ofquantum dot materials, as well as the size range of the quantum dots,different emission spectral profiles can be obtained. Similar totraditional phosphor materials, quantum dots are excited by illuminatingthem with shorter wavelength light. As with traditional phosphormaterials, quantum dots with a variety of excitation wavelengths andemissions spectra are commercially available. One key difference betweenquantum dots and traditional phosphor materials, however, is thatquantum dots are commercially available with emission spectra in theblue range of visible light, as well as in the green, yellow, and redranges. That difference notwithstanding, wherever phosphors arementioned in the description of the present invention, it is alsopossible to substitute quantum dots for the phosphor material.

In one embodiment of the invention, a compact illuminating apparatuscomprises an LED array with accompanying optical elements that outputsintense, broad-spectrum light uniformly over a field of view. The LEDarray contains multiple strings, each comprising several LED chips ofpotentially different wavelengths, with each string controlledelectronically as a separate channel. The LED chips are of wavelengthsthat are able to excite emissions from commercially-available phosphormaterials. Phosphors of multiple types and emissions spectra aredispensed or applied on top of all of the individual LED chips, suchthat it is possible to have a different phosphor type on each individualLED chip, or on different subsets of the LED chips. In addition to theuse of traditional phosphor materials, it is also possible to use othermaterials with luminescent properties, such as quantum dots. Themultiple channels allow for each string of LEDs and hence their outputcolor and power to be independently switched on/off and varied inintensity, respectively. This allows the LED illuminator to providevariable or adjustable color temperature (CCT), while maintainingextremely high CRI. The pattern of LED wavelengths and differentphosphor types (or other luminescent materials) and emissions spectra isconfigured for high color uniformity. The optical elements immediatelysucceeding the LED array serve to collect and reshape the output lightto enhance both light coupling efficiency and uniformity. A lensattached to the surface of the LED array enhances light extraction, andsecondary optics including reflectors, additional lenses, and/ordiffusers may be used to further shape the beam, and to further improvethe homogeneity and uniformity of the beam, both in terms of lightoutput and color.

In another embodiment of the invention, a method for achieving highphosphor excitation efficiency is also introduced. A transparentmaterial with high index of refraction, such as a silicone gel, is usedto fill around the edges of the multiple LED chips, to increase theescape of light from the sides of the LED chips' “slab” shape. Siliconegel containing potentially differing phosphor types is then applied ontop of each chip, in such a way that there are no gaps or voids in theoverall coverage of the LED chips by phosphor-containing gel. Differentphosphor types may be dispensed on different subsets of the LED chips inthe LED array. This process may also be used with other luminescentmaterials, such as quantum dots. The above embodiments are described indetail below.

FIGS. 6A and 6B show front and side views, respectively, of one possibleembodiment of an LED array of the present invention. The LED arraycomprises a multiplicity of individual LED chips (6A01 and 6B01). TheLED array embodiment depicted in FIGS. 6A and 6B contains 60 LED chips,although in other embodiments the number of LED chips may be as small as4, or in excess of 100. The individual LED chips of the embodiment shownin FIGS. 6A and 6B are approximately 1 mm×1 mm in size. However, LEDchips of other dimensions may also be used, and the diameter of thelight-emitting area is therefore a function of the number of LED chips,the individual chip dimensions, and the spacing between LED chips. InFIG. 6A, the LED chips are shown as element (6A01), with each smallsquare representing an individual LED chip. The diameter of thelight-emitting area of the array is approximately 5-25 mm, depending onthe number of LED chips in the array, as well as the individual chipsize and spacing, thereby allowing the LED array to function as an“extended point source”. Note that the single lens that is placed overthe light-emitting area of the LED array (6B03) may have a diameter thatis slightly larger than the actual light-emitting area, such as adiameter of about 6-30 mm.

The LED chips within each array are connected electrically into multiplechannels, each channel consisting of at least one LED chip, or a seriesstring of multiple LED chips. An LED string or channel is controlled asa single entity, with all LED chips within the series string having anidentical electrical current passing through them, and therefore eachchip within a string will produce light of similar brightness. Separateelectrical connections or connectors (6A02) are provided for each LEDstring within the array, either in the form of electrical pins, or aselectrical pads, as shown in the figure, so that the relative brightnessof the different strings can be controlled and varied independently ofone another. The embodiment shown in FIGS. 6A and 6B comprises eightchannels or strings of LED chips, with the channel number shown withineach LED chip (6A01), and also adjacent to the electrical connections(6A02). Note that the number of channels may vary, as described in moredetail below. Also, channels may be electrically connected to eachother, external to the array, if fewer separately-controllable channelsare required. For example, although the embodiment shown in FIGS. 6A and6B comprises eight channels of LED chips, the driver circuit for theembodiment might comprise four driver circuits, each of which is drivingtwo LED strings. Typically, the multiple LED channels that are to bedriven by a single driver circuit would be connected in series, suchthat the drive current in the combined channel would be constant. TheLED array shown in FIGS. 6A and 6B also incorporates one or moreinternally-mounted thermistor chips, which are brought out to some ofthe unlabeled electrical connections, for the purposes of monitoringarray temperature.

Thermal management is a key element of the design of the presentinvention, in order to extract the heat that is generated by the largenumber of LED chips that are packaged closely together in the LED array.The LED array incorporates a metal circuit board (MCB) which providesfor the routing of conductive traces to each of the LED strings, whileat the same time providing electrical isolation between LED strings. TheMCB also provides for high thermal conductivity, to extract heat fromthe densely-packed LED chips. The MCB LED array substrate, having thethermal conductivity of metal, conducts the heat from the LEDs to thebase of the MCB substrate, which is mounted onto a heat spreader or heatsink. The MCB of the present invention is described in more detail inU.S. Pat. No. 8,044,427, issued on Oct. 25, 2011, entitled “LIGHTEMITTING DIODE SUBMOUNT WITH HIGH THERMAL CONDUCTIVITY FOR HIGH POWEROPERATION”.

In most embodiments of the present invention, the LEDs within a stringwould be of similar wavelengths. However, different strings mightcontain LEDs of varying wavelengths. In one embodiment of the LED array,some strings would consist of different wavelengths of blue, indigoand/or violet light, with peak wavelengths that are less than 490 nm.Similarly, an individual LED string or channel might use just a singlephosphor type, or it might use multiple phosphor types. Note that in allcases, all of the LED chips of the array are coated by one or morephosphor types, with different individual LED chips, even adjacentchips, having potentially different phosphor types coated onto them. Theapplication process for applying phosphor materials to the LED chipsallows for a differing, or even a unique phosphor formulation, to beapplied at each one of the multiple LED chip sites. The phosphorformulation on any given chip may consist of a single type of phosphormaterial, mixed into a silicone gel material, or a blend of multipletypes of phosphor materials mixed into the gel, for an even broaderspectrum. By separately controlling the electrical current flowingthrough different LED strings, the relative proportions of light ofdifferent spectral characteristics can be varied. Within a particularstring, it is still possible to use individual LED chips of multiplewavelengths, although the intensity of the light emission of the LEDchips within a string will be of the same order of magnitude. In oneembodiment of the present invention, multiple wavelength ranges of blueand indigo light (for example, LED chips with peak wavelengths atapproximately 430 nm and 455 nm) could be used, in order to achievebroader coverage of the blue portion of the spectrum, as well as optimalexcitation of specific phosphor types. Similarly, multiple wavelengthranges of phosphors, such as predominantly green, yellow, andred-emitting phosphors could be used either within a string, or inmultiple strings, in order to achieve broader coverage of the green,yellow and red portions of the spectrum.

FIGS. 7A and 7B show one embodiment of the LED illuminator of thepresent invention, showing an exploded view of most of the illuminatorapparatus, without its outer housing. FIGS. 7A, 7B and FIG. 8 representjust one of the possible form factors for an illuminator apparatus ofthe present invention. In FIG. 7A, the LED array (701) is mounted onto aheat spreader (702). As discussed above, the Metal Circuit Board (MCB)of the LED array provides for efficient heat transfer from the LEDchips, to the heat spreader. In most embodiments of the illuminatingapparatus, a heat sink of some kind will be mounted underneath the heatspreader (702). The heat sink of this embodiment is omitted from FIG.7A, to show that in this embodiment there is a fan (704) underneath.(The heat sink is shown in FIG. 8.) Beneath the fan (704) is a printedcircuit board or PCB (705) containing the control and drive electronicsfor the apparatus. In order to shape the light output of the LED array,secondary optics may be designed into the illuminating apparatus. In theembodiment shown in FIG. 7B, beam shaping is provided by a reflector(706). More elaborate optics, including additional lenses, apertures,shutters, zooming mechanisms, etc., may also be used.

FIG. 8 shows additional details of the illuminator apparatus,representing one embodiment of the present invention. The heat sink(803) is shown mounted between the heat spreader and fan that were shownin FIG. 7A. In order to provide even better color-mixing, uniformity,and homogeneity of the light output, a diffuser element (807) may beincluded. One embodiment of a simple outer housing (808) is shown,without design features.

Although not shown in any of the figures, a complete illuminatorapparatus will have one or more control knobs or other forms of usercontrol(s). In one embodiment, one knob might be used to control overallbrightness of the illuminator apparatus, with a second knob being usedto control CCT. Other functional assignments of control knobs are alsopossible, such as having one knob control the brightness of a “warmwhite” light output of low CCT (such as 2500K), while a second knobcontrols the brightness of a “daylight” light output of higher CCT (suchas 6500K). Varying the settings of the two control knobs would thuscreate a light output with CCT that is somewhere in the range of 2500Kto 6500K. Other control implementations are within the scope of thepresent invention.

FIG. 9A shows a spectral plot of one embodiment of the presentinvention, based on the LED array embodiment depicted in FIGS. 6 and 10.The LED array of this embodiment comprises two wavelengths of indigo andblue LEDs, with spectral peaks at approximately 430 nm (9A01) and 455 nm(9A02), respectively. The LED chips of this embodiment are coated withthree types of phosphors, that absorb energy from either or both of theblue LED chip types, and emit broad spectrum light with emissions thatare roughly in the green (9A03), yellow (9A04), and red (9A05) portionsof the visible spectrum. It should be noted that all of the LED chips ofthis embodiment are coated with at least one of the phosphor types.Applying a mixture of phosphor types onto a given LED chip, or set ofLED chips, will result in a broader spectral output from thoseparticular chip location(s), potentially improving the color and hueuniformity within the field of view of the illuminator. As notedpreviously, the application process for the phosphors allows differingphosphor types or mixtures of phosphor types to be applied at differingLED chip sites, even to the extreme of having a unique phosphor mixturefor every LED chip (that is, the phosphor mixture on top of or aboveeach LED being unique and different from the phosphor mixture on top ofor above any other LED). Although the embodiment depicted in FIGS. 9Aand 9B uses two wavelengths of indigo and blue LEDs, other embodimentsmight use just a single wavelength of blue, indigo, or violet LED, ormore than two wavelengths. Similarly, the number of phosphor types maybe less than, or more than, three.

As shown in FIG. 9A, the combined spectral output of the blue LEDwavelength(s), and the multiple phosphor types, results in a spectralplot that stays close to that of an ideal black body radiator (9A06),with both good spectral fill (i.e., without deep gaps in the spectrum),and also the avoidance of spectral “hot spots”, due to excessive outputat specific wavelengths. The fact that all of the LED chips of thisembodiment are coated in phosphor, significantly reduces the potentiallyexcessive light output at specific narrow wavelength ranges and thusavoids spectral “hot spots” at such narrow wavelength ranges, that wouldresult from having uncoated LED chips. Also, the use of multiplewavelengths of blue (or multiple wavelengths of blue, indigo and/orviolet) LED chips, all of which serve to excite emissions from all ofthe phosphor types used, also serves to avoid excess light output atspecific narrow wavelength ranges, in comparison to simply using asingle wavelength of blue LED chip. The CRI of the present inventioneasily and routinely exceeds 95. Through careful selection of thephosphor types, as well as careful control of the application process,CRI values as high as 99 are achievable.

The embodiment of LED array whose spectral plot is shown in FIG. 9A maybe of either fixed, or adjustable CCT, although the plot of FIG. 9Ashows a representative CCT of 4500K. The CIE chromaticity diagram ofFIG. 9B shows that this embodiment of the present invention also hascoordinates that lie on the ideal black body radiator curve, atdiffering possible CCT values.

In order to for the embodiment of LED array to have adjustable CCT,while maintaining CIE chromaticity diagram color coordinates that lie onor close to the ideal black body radiator curve for all of its intendedCCT values, the array is designed such that its constituent LED strings(or groups of LED strings) are separately controllable. Each separatelycontrollable LED string or group of strings will have its own colorcoordinates, that may be plotted onto the CIE chromaticity diagram. Bycontrolling the relative brightness of the multiple LED strings (orgroups of strings), any color or hue whose coordinates lie within thebounds of the individual strings' color coordinates can be achieved. Forexample, if there are three separately controlled LED strings, each withits own blend of applied phosphors and its own CCT value and differentrespective CIE chromaticity diagram color coordinates, then any desiredCCT value and color or hue can be created, such that the resulting colorcoordinates lie within the triangle that is formed by the respectivecolor coordinates of the three strings. This is similar in concept tothe creation of multiple colors that fall within the “color triangle” ofan RGB LED array, except that in the present invention the individualstrings have color coordinates that lie fairly close to the ideal blackbody radiator curve to begin with, and therefore would tend to have thevisual appearance of varying shades or hues of generally white light.Thus, if the three LED strings of this embodiment of the presentinvention are plotted onto the CIE chromaticity diagram, the resultingtriangle will be relatively small (in comparison to an RGB LED array'scolor triangle), and will be centered on the ideal black body radiatorcurve.

In another embodiment of the present invention, four or more separatelycontrollable LED strings (with applied phosphors) are used. Thesestrings are configured such that their respective CIE chromaticitydiagram color coordinates form a rectangle or parallelogram thatstraddles a section of the ideal black body radiator curve.Specifically, one string has color coordinates that fall somewhat abovethe high-CCT end of the ideal black body radiator curve (the “upperleft” corner of the rectangle or parallelogram). The second string hascolor coordinates that fall somewhat below the high-CCT end of the idealblack body radiator curve (the “lower left” corner of the rectangle orparallelogram). Similarly, the third and fourth strings have colorcoordinates that bracket the low-CCT end of the ideal black bodyradiator curve. By varying the relative intensity or brightness of thefour strings, the coordinates of the resulting light can be placed atany desired CCT value on the ideal black body radiator curve that fallswithin the rectangle or parallelogram.

In practice, the color coordinates of any individual LED string willhave some variability. By spacing the color coordinates of the multipleLED strings far enough apart (in terms of their respective positions onthe CIE chromaticity diagram), the variability of the individual stringscan be corrected by carefully balancing the relative intensity orbrightness of the multiple LED strings. In order for the CRI value ofthe resulting combined light to be high, in excess of 95, the CRI of theindividual LED strings needs to be high as well. However, in as much asthe individual LED strings have differing spectral profiles, the CRIvalue of the combined light output will in general be higher than theCRI values of the individual LED strings.

Although the dimensions of the light-emitting area of the LED array ofthe present invention is small, it still has finite area. Since the LEDarray comprises LED chips of multiple wavelengths, coated with phosphorsof multiple wavelength ranges, differing colors of light output fromdifferent areas of the array surface may still cause non-uniformity inthe color and hue of the resulting output light spot, or beam. In orderto achieve good color/hue uniformity and homogeneity of its light outputwithin the illuminated area, the LED array of the present invention isconfigured so that the various types of LED chips and phosphors aredispersed over the area of the array, such that the different types ofLED chips and phosphors are intermingled with each other. In addition,the array is configured to be symmetrical around several axes. Thissymmetry, when combined with the effects of secondary optics such as adiffuser, light pipe, and/or a corrugated or textured reflector, servesto increase the spatial uniformity of the beam, in terms of its colorand hue.

FIGS. 10A and 10B show one embodiment of the LED array of the presentinvention, using two wavelengths of blue and indigo LED chips, coatedwith three types of phosphors, and therefore similar in configuration tothe embodiment whose spectral plot was depicted in FIG. 9A. FIG. 10Ashows the arrangement of two wavelengths of LED chips, designated as“blue” for LED chips with a spectral peak within an approximatewavelength range of 450-490 nm, and as “indigo” for LED chips with aspectral peak within an approximate wavelength range of 410-450 nm. Thespecific wavelengths used may vary, but in one embodiment the “blue” LEDchips would have a spectral peak at approximately 455 nm, and the“indigo” LED chips would have a spectral peak at approximately 430 nm.Note that the “blue” and “indigo” chip locations are intermixed, andthat there is four-fold symmetry around the four axes of symmetry shownin the figure. In other words, for each LED chip type or each peakwavelength emitted by the LEDs, there are equal quantities of that chiptype or LEDs emitting light of such peak wavelength on both sides ofeach of the four axes of symmetry, including the vertical axis V, thehorizontal axis H, and both diagonals D1, D2 of the LED array. Thefour-fold symmetry of the LED chips and of the phosphor types describedbelow, is defined as having equal quantities of that chip or phosphortype on both sides of each of the four axes of symmetry, including thevertical axis V, the horizontal axis H, and both diagonals D1, D2 of theLED array. In other words, the LED chip type (or LED emitting such peakwavelength) and phosphor type do not need to be mirror images across thefour axes. Similarly, FIG. 10B shows the arrangement of three phosphortypes, one of which is applied to the top of each of the LED chips. Thenumber of phosphor types may be greater than or less than three, but inone embodiment, the three phosphor types P1, P2, and P3 would havespectral emissions that would be generally in the yellow, green, and redportions of the visible light spectrum, respectively. Note that the chiplocations of the phosphor types are intermixed, and that there is alsofour-fold symmetry of the phosphor configuration. In this embodiment,all of the phosphor types are excited by either of the LED chip types.This LED chip and phosphor configuration provides excellent light mixingin terms of uniformity and homogeneity, which may be further augmentedby the incorporation of a diffuser element or other secondary opticswithin the overall optical design of the LED illuminator.

Referring back to the embodiment of the present invention shown in FIG.6A, it should also be noted that the LED chip locations for the multipleLED strings of the array (as denoted by the string numbers within eachchip location) have symmetry with respect to the four axes of symmetry.The strings are laid out as symmetric pairs, such that string 1 andstring 5 form a symmetric pair, string 2 and string 6 form a symmetricpair, string 3 and string 7 form a symmetric pair, and string 4 andstring 8 form a symmetric pair, all with respect to the center 6A06 ofthe array. String 1 and string 7 form a symmetric pair, string 2 andstring 8 form a symmetric pair, string 3 and string 5 form a symmetricpair, and string 4 and string 6 form a symmetric pair, all with respectto the horizontal axis H. String 1 and string 3 form a symmetric pair,string 2 and string 4 form a symmetric pair, and string 5 and string 7form a symmetric pair, and string 6 and string 8 form a symmetric pair,all with respect to the vertical axis V. In addition, the LED chiplocations within each string are symmetric with respect to one of thediagonal axes D1, D2 of symmetry. Other embodiments with differentnumbers of LED chips and different numbers of LED strings are possible,while maintaining symmetry with respect to the four axes of symmetry.

The present invention proposes the application of different phosphormaterials over the tops of individual LED chips, which are spacedtightly together in an LED array. The phosphor material (typically of asingle phosphor type, although in some embodiments multiple phosphortypes could be mixed together) is mixed into a silicone gel material,which is dispensed as a viscous liquid, and then cured to become solid.The silicone gel is of high viscosity, and the dispensed quantity iswell controlled, so that each LED chip is fully covered by the siliconegel with its intended phosphor type(s), without spreading to coveradjacent LED chips, since it may be desirable to apply a differentphosphor type to these adjacent LED chips. Commercially availablesilicone gels for LED packaging applications are designed to beoptically transparent throughout the visible light range of wavelengths,extending down to the UV range. They are also designed to have a wellcontrolled index of refraction, for good light extraction from thesurface of the LEDs.

FIG. 11 shows a cross-section view of a small portion of one embodimentof the LED array of the present invention, showing several of the LEDchips (1101) in cross-section, prior to the application of the siliconegel and phosphor materials. This figure is not drawn to scale. As statedpreviously the lateral dimension of each LED chip is typically around 1mm, although different LED chip sizes are possible. The space or gapbetween chips would generally be in the range of 100 μm to severalhundreds of μm. The LED chips are bonded to the Metal Circuit Board(MCB) substrate (1102) using metallic solder (1103), for good thermaltransfer. The arrow symbols represent light emission from within the LEDchips. Light that is emitted by the active light-emitting epi-layer ofthe LED chips can escape the chip boundaries at both the top surface ofthe LED chip, and at its side surfaces, as long as the angle ofincidence with the LED surface is within the critical angle that isdefined by the relative indices of refraction of the LED chip materialand the surrounding air. Without delving into the optics and physicsinvolved, the net effect is that much of the light that is emitted bythe LED's active light-emitting region can become “trapped” within theLED chip, due to internal reflection at the interfaces between the chipsurface and the air. FIG. 11 shows a crude illustration of thisphenomenon. This serves to reduce the efficiency of the LED's lightemission, resulting in both lower light output and increased heatgeneration within the LED chip.

If a silicone gel that contains phosphor material is deposited only onthe top surface of the LED chip, then some of the LED light emission outof the side walls of the chip will bypass the silicone gel and phosphormaterial. This can lead to excessive output at the LED chip'swavelength, and reduced output from the phosphor material, therebycreating “hot spots” in the spectral plot of the LED array, with reducedCRI. It also results in reduced excitation of the phosphor material,resulting in reduced light output.

In order to both maximize the overall light output of the LED array, andalso to avoid spectral “hot spots” caused by gaps in the coverage of theLED chips by the phosphor-containing silicone gel, the silicone gel andphosphor materials are applied to the LED array of the present inventionin a two-step process, as depicted in FIG. 12. First, anoptically-transparent silicone gel, without phosphors, is applied to thearea (1204) between LED chips (1201), filling these areas (1204) up tothe level of the top surfaces of the LED chips. The silicone gel isselected to have a high index of refraction, in order to maximize theemission of light through the side walls of the LED chips, and into thearea of transparent silicone gel. Then, the multiple types of phosphormaterials are applied to the tops of the designated individual LEDchips, carried in an optically-transparent silicone gel (1205). Thephosphor types that are used on relatively small numbers of chip siteswould generally be applied first. The phosphor types that are used onlarger numbers of chip sites are therefore applied last. However, theexact order of applying the different phosphor types is not critical.When the final (or later) phosphor types are being applied, thedispensed quantity of silicone gel (with phosphor material inside) isadjusted to ensure full coverage of the surface of the LED array,including coverage of the areas (1204) between LED chips that havepreviously been filled with transparent silicone gel, such that thereare no gaps between LED chips where there is no phosphor-containingsilicone gel. In other words, the silicone gel (with phosphor materialinside) 1205 applied also covers the optically-transparent silicone gel,without phosphors, that has already been applied to fill the gaps 1204between the LED chips, so that there is substantially no gap between thesilicone gel (with phosphor material inside) 1205 that is applied on topof or above the LED chips.

The solid arrow symbols shown in FIG. 12 represent the light emissionfrom the LED chips. The dashed arrow symbols represent the lightemissions from the phosphor material that is embedded within the toplayer of silicone gel. As depicted in FIG. 12, this phosphor andsilicone gel application process results in good transfer of LED lightemissions into the area of the silicone gel material that containsphosphors, such that the light emission from the phosphors is maximized.Some of the light emitted by the LED chips passes through the siliconegel area with phosphors, adding to the overall spectral plot of the LEDarray. However, voids or gaps in the phosphor-containing silicone gellayer are avoided, thereby minimizing spectral “hot spots” at the LEDchips' wavelengths. This contributes to the LED array of the presentinvention possessing the desired attributes of both high efficiency, aswell as high color/hue uniformity and homogeneity of its light output.Note also that the MCB (1202) substrate's top surface (1206), and themetallic solder layer (1203) that is underneath the LED chips, are bothreasonably reflective in the visible light range, such that light thatis directed downward will largely be reflected back upwards, therebyfurther increasing the overall light output from the LED array.

In order to vary the CCT of the LED illuminator of the presentinvention, it is necessary to provide an electronic control circuit thatcan independently control multiple channels or strings of LED chips,within the LED array. As shown in FIG. 6A, and in FIGS. 10A and 10B, themultiple LED channels or strings of the LED array of the presentinvention utilize LED chips with different wavelengths, and phosphormaterials with different emissions spectra. The multiple LED channels orstrings will therefore have differing CCT values when independentlydriven. By altering the relative brightness of the multiple LED channelsor strings, the CCT of the combined light output may be adjusted, over abroad range of values.

FIG. 13A shows one embodiment of an electronic control circuit for theLED illuminator of the present invention. In this embodiment, themultiple LED channels or strings of the LED array are combined into fourLED driver circuits, with a pair of manual control knobs to vary thebrightness level of two sets of driver circuits. By connecting orconfiguring the LED array channels or strings with high CCT such thatthey are controlled by one of the manual control knobs, and connectingthe LED array channels or strings with low CCT such that they arecontrolled by the other manual control knob, it is possible to achievean overall CCT value that is anywhere between the CCT values of the twoseparately-controlled subsets of the LED array. For one skilled in theart of designing electronic control circuits for LEDs, it will beobvious that different implementations of driver circuits and controlscan be used for additional embodiments of the present invention, toaccount for LED arrays with different numbers of LED chips, differentnumbers of separately-controllable LED channels or strings, anddifferent configurations of the multiple LED wavelengths and phosphortypes.

FIG. 13B shows another embodiment of an electronic control circuit forthe LED illuminator of the present invention. In this embodiment, amicrocontroller is used to control the four LED driver circuits. Thismicrocontroller can interpret or accept control inputs from a variety ofinput types, including one or more state selector switches, one or morebrightness control knobs, one or more CCT control knobs, serial controlinterfaces, or wireless control interfaces. Using information from oneor more of these control interfaces, the microcontroller controls thebrightness of the multiple LED driver circuits such that both theoverall brightness and CCT of the LED illuminator are determined.

In one embodiment of the LED illuminator, using either of the electroniccontrol embodiments shown in FIG. 13, or some other electronic controlembodiment, the CCT of the combined light output of the LED array of thepresent invention can be adjusted from 2500K to 6500K, thereby spanningthe CCT range from tungsten-filament incandescent light bulbs, tonatural sunlight. This could be done by having a greater concentrationof red phosphors in some of the LED channels or strings, to producelight with a CCT value of 2500K, and greater concentration of greenphosphors (and to some extent yellow phosphors) in other LED channels orstrings, such that they produce light with a CCT value of 6500K. An evenbroader range of CCT values is possible, for example from 2000K to10,000K, with the 10,00K CCT value being achieved by using less phosphormaterial over the blue and indigo LED chips, such that relatively moreblue light emerges. However, this broader range of CCT values would beaccompanied by a reduction in the maximum light output obtainable at anygiven CCT value. For most applications in the fields of photography,videography, and motion picture lighting, the desired CCT range of alight source is usually within the range of 2500K to 6500K.

As discussed above, in order to ensure that the resulting combined lightoutput has CIE chromaticity diagram color coordinates that lie on, orclose to the ideal black body radiator curve, for all of the desired CCTvalues, it will usually be necessary to have at least three, andpreferably four or more separately-controllable LED strings, withrespective individual color coordinates that bracket or enclose thedesired range of CCT value on the ideal black body radiator curve. Thiswill allow for some variability in the color coordinates of theindividual LED strings.

Since all of the LED channels or strings of the LED array of the presentinvention are implemented using a mix of one or more LED wavelengths andmultiple phosphor types, extremely high CRI is maintained over the fullrange of CCT adjustment. This is in contrast to the prior art method ofusing red LEDs to “warm” the light output and thereby vary CCT. In theprior art method for adjusting or varying CCT, the spectral “hot spot”created by simply adding light from plain red LED chips can actuallymake the CRI value of the prior art illuminator worse, as its CCT isadjusted.

While the invention has been described above by reference to variousembodiments, it will be understood that changes and modifications may bemade without departing from the scope of the invention, which is to bedefined only by the appended claims and their equivalents.

The invention claimed is:
 1. An illumination apparatus, comprising: an LED array, said array including three or more strings of LEDs, wherein said strings of LEDs emit light of one or more wavelengths of blue, indigo and/or violet light, with peak wavelengths that are less than 490 nm; said LED array comprising bare LED chips that are mounted in close proximity to each other on a substrate; luminescent materials that are deposited on each of the LED chips in the array, said materials emitting light of different wavelength ranges that are of longer wavelengths than said light emissions from the LED chips, with the emissions from the luminescent materials being in response to such light emissions from the LED chips; a control circuit that applies currents to the strings of LEDs, causing the LEDs in the strings to emit light, which causes the luminescent materials to emit light; and a user interface that enables users to control the currents applied by the control circuit to the strings of LEDs to achieve a Correlated Color Temperature (CCT) value and hue that are desired by users, with CIE chromaticity coordinates that lie on, or near to the black body radiation curve, wherein the different peak wavelengths of the LEDs in the array are arranged symmetrically about two orthogonal axes, and about two diagonal axes at 45 degrees to the two orthogonal axes, so that for each peak wavelength of LEDs in the array, there are equal quantities of LEDs with such peak wavelength on both sides of each of the four axes.
 2. An illumination apparatus, comprising: an LED array, said array including three or more strings of LEDs, wherein said strings of LEDs emit light of one or more wavelengths of blue, indigo and/or violet light, with peak wavelengths that are less than 490 nm; said LED array comprising bare LED chips that are mounted in close proximity to each other on a substrate; luminescent materials that are deposited on each of the LED chips in the array, said materials emitting light of different wavelength ranges that are of longer wavelengths than said light emissions from the LED chips, with the emissions from the luminescent materials being in response to such light emissions from the LED chips; a control circuit that applies currents to the strings of LEDs, causing the LEDs in the strings to emit light, which causes the luminescent materials to emit light; and a user interface that enables users to control the currents applied by the control circuit to the strings of LEDs to achieve a Correlated Color Temperature (CCT) value and hue that are desired by users, with CIE chromaticity coordinates that lie on, or near to the black body radiation curve, wherein the different luminescent materials with different emissions spectra are arranged symmetrically about two orthogonal axes, and about two diagonal axes at 45 degrees to the two orthogonal axes, so that for each type of luminescent material with a particular emissions spectrum, there are equal quantities of LED chips with such luminescent material deposited on said LED chips, on both sides of each of the four axes.
 3. An illumination apparatus, comprising: an LED array, said array including three or more strings of LEDs, each of said strings comprising multiple LEDs, wherein said strings of LEDs emit light of one or more wavelengths of blue, indigo and/or violet light, with peak wavelengths that are less than 490 nm; said LED array comprising bare LED chips that are mounted in close proximity to each other on a substrate; luminescent materials that are deposited on each of the LED chips in the array, said materials emitting light of different wavelength ranges that are of longer wavelengths than said light emissions from the LED chips, with the emissions from the luminescent materials being in response to such light emissions from the LED chips; a control circuit that applies currents to the strings of LEDs, causing the LEDs in the strings to emit light, which causes the luminescent materials to emit light; and a user interface that enables users to control the currents applied by the control circuit to the strings of LEDs to achieve a Correlated Color Temperature (CCT) value and hue that are desired by users, with CIE chromaticity coordinates that lie on, or near to the black body radiation curve, said LED array having a center and two orthogonal axes wherein the different strings of LEDs are arranged or configured as symmetric pairs, with respect to the center of the LED array, and wherein the LED chip locations of each LED string are arranged or configured symmetrically around one of the two orthogonal axes of the LED array. 